Watch Talk: What’s the Beat Error of a Watch?
The beat error is a hot item on many watch-related forums.
If you look it up, you might find something like: “beat error is an asymmetrical oscillation of the balance wheel.”
In this article, I’ll explain what it is and why it’s important.
What is it?
As you know, the balance wheel swings clockwise and counterclockwise. Each one of those swings, in either direction, is called a “beat.” If the watch runs perfectly, the clockwise swing will be the same as the counterclockwise swing, and both swings will take the same amount of time.
The beat error is the difference between the clockwise swing and the counterclockwise swing of the balance. In perfect conditions, the beat error is 0.
However, perfect conditions don’t exist in the real world.
In reality, the clockwise swing, for example, will take a bit longer than the counterclockwise swing. The watch is slightly “out of beat.”
In the example above, the beat error could be 1.2 ms (it’s measured in milliseconds). That means that the clockwise swing takes 1.2 ms longer than the counterclockwise swing.
Do you have a watch that doesn’t start ticking on its own but needs some “help”? That might be due to a high beat error.
What causes it?
The balance wheel swings to the left and the right. Halfway, the impulse pin of the balance enters and exits the fork of the lever.
The lever is placed between two banking pins.
The zero position of the balance is the position in rest without any power in the mainspring. The location of the impulse pin is most important because that’s the middle point between a clockwise swing and a counterclockwise swing.
The dead center of the banking pins is the ideal resting place for the impulse pin in the zero position. The beat error would be 0.
However, it’s nearly impossible to achieve this so the impulse pin will likely be a little to the left or right of this point. That causes a beat error because one direction of the swing is slightly longer than the other one.
Remember that we’re talking milliseconds here, so the differences are minimal.
How do you correct it?
Mobile stud carrier
If you’re lucky, your watch has a mobile stud carrier. With it, you can quickly reposition the complete balance and therefore the position of the impulse pin in relation to the banking pins.
Place your watch on a timing machine so you can manipulate the mobile stud carrier and monitor the beat error at the same time.
Fixed stud carrier
If the movement doesn’t have a mobile stud carrier, you’ll need to move the collet of the hairspring on the balance staff. That will change the position of the impulse pin in the zero position as well.
Always remove the balance and place it on a balance tack if you want to move the collet. You can use a tiny screwdriver, or you can use a special hairspring collet tool like the Bergeon 30017.
As you can imagine, this is a very fiddly job so you should only consider this when it’s necessary. Otherwise, it’s not worth the risk of damaging the hairspring, especially if you know that spare parts are impossible to find.
What’s a good beat error?
A watch with a high beat error can still keep accurate time if it has a strong amplitude and a consistent daily rate. However, it’ll be more vulnerable to problems with timekeeping when the amplitude is low or during sudden changes in the position of the watch.
If your movement has a mobile stud carrier, you can easily adjust the beat error to 0.2 ms or 0.3 ms. If it has a fixed stud carrier, a beat error of 0.8 ms and below is fine.
Have you ever corrected the beat error? If you have any more tips, please leave them in the comments below.